FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an apparatus for generating an ultrasonic oscillation, and more particularly to an apparatus for generating an ultrasonic oscillation comprising an ultrasonic transducer having an ultrasonic vibrating element for producing an ultrasonic oscillation and a probe for transmitting the oscillation produced by the ultrasonic vibrating element, and a driving circuit for supplying a driving signal to the ultrasonic vibrating element.
Heretofore, there have been proposed various kinds of apparatuses using the ultrasonic transducer.
For instance, ultrasonic surgical knives and ultrasonic working machines have been developed. In these ultrasonic apparatuses, it is advantageous to effects the impedance matching between the ultrasonic transducer and, the driving circuit in order to improve the driving efficiency of the ultrasonic vibrating element.
FIGS. 1A and 1B show the known ultrasonic probe in which vibrating rods 2 and, 3 having different lengths are detachably secured to an ultrasonic transducer 1. The inventor of the instant application has experimentally confirmed that the impedance of the probe illustrated in FIG. 1B is smaller than that of the probe depicted in FIG. 1A by about five times. Therefore, when these probes are driven by the same driving circuit, if the output impedance of the driving circuit is fixedly matched to either one of the vibrating rods 2 and 3, the impedance matching could not be attained for the other of the vibrating rods 3 or 2 and the driving efficiency of the ultrasonic vibrating element 1 would be decreased to a great extent.
In order to mitigate the above mentioned drawback, in a Japanese Patent Laid-open Publication Kokai Sho 63-162086, there has been proposed an ultrasonic transducer in which taps on a secondary side of a coupling transformer for electromagnetically coupling the ultrasonic vibrating element and the driving circuit are switched in accordance with the amplitude of the oscillation or vibration of the ultrasonic vibrating element.
However, in the ultrasonic oscillation generating apparatus disclosed in said Japanese Patent Laid-open Publication Kokai Sho 63-162086, there is a problem that the impedance matching could be no more attained due to the fact that the adjustment of the impedance matching is carried out on the basis of the amplitude of the oscillation. That is to say, in the ultrasonic transducer the amplitude of the oscillation is proportional to an amplitude of a current passing through the ultrasonic vibrating element, so that if the impedance of the transducer circuit is increased twice, the taps on the secondary side of the coupling transformer are changed such that the voltage applied to the ultrasonic vibrating element is increased also twice in order to keep the amplitude of the current unchanged. Then, the impedance of the transducer circuit viewed from the primary side of the transformer is decreased by four times, because the ratio of the primary winding to the secondary winding becomes 1:2. This results in that the impedance of the load for the driving circuit is decreased by two times, although the impedance of the transducer circuit is increased by two times. Therefore, the impedance matching could not be attained and the ultrasonic vibrating element could not be driven efficiently.
Further, in the known ultrasonic generating apparatus, the taps are provided on the secondary side of the coupling transformer, i.e. on the vibrating element side of the transformer. When the apparatus is applied to the ultrasonic surgical knife, a switching circuit for switching the secondary winding portions is arranged in the circuitry on the patient side, so that the electrical insulation should be effected to a very high degree in order to achieve the protection against the electric leakage and discharge. This apparently increases the cost of the apparatus.
In order to attain a proper impedance matching, it would be also considered that the impedance of ultrasonic transducers to be used are previously measured and when an ultrasonic transducer is used, the impedance matching is attained manually in accordance with the measured impedance of the relevant transducer. In such a solution, there might be produced another problem of the misoperation of the user and the driving circuit might be broken under the overload condition.
In Japanese Patent Laid-open Publication Kokai Sho 63-212341 and 63-212342, there are described further known ultrasonic apparatuses in which objects such as hematoma and tumor produced within a patient body are broken into pieces by irradiating the ultrasonic beam thereupon by inserting the ultrasonic endoscope and pieces of the objects are sucked out of the body via a tube arranged in the endoscope. In such ultrasonic surgical operating apparatus, it is desired that the amplitude of the ultrasonic probe driven by the ultrasonic vibrating element is kept constant regardless of the acoustic impedance of the objects. As explained above, since the amplitude of the ultrasonic vibrating element is proportional to the amplitude of the current passing through the element, the output of the oscillator in the driving circuit is supplied to the vibrating element via a voltage controlled amplifier (VCA) whose amplification factor can be changed by a control voltage, and the amplification factor of the VCA is adjusted in accordance with the driving current such that the driving current can be kept constant.
In the above mentioned ultrasonic apparatus in which the ultrasonic vibrating element is driven by the constant current circuit, the construction of the apparatus can be made simple and the amplitude of the ultrasonic oscillation can be maintained substantially constant. However, the electric stability of the known apparatus sometimes becomes deteriorated. For example, in the above explained ultrasonic surgical apparatus, when the tip of the ultrasonic probe is urged against the object, the electric property of the probe is changed to a large extent in accordance with the objects and the amplitude of the driving current becomes extremely small. Then, the constant current circuit operates such that the control voltage for the VCA is abnormally increased and the voltage applied to the ultrasonic vibrating element becomes larger than threshold voltages of the element and driving circuit, so that they might be broken. This is quite dangerous for the patient.